1994 Rialta in Santa Cruz
We had a similar problem i.e. the pump ran but nothing happened.
There is an elbow in the pipework that the silt collects in and solidifies.
I had to take the back panel off behind the loo and expose all the pipe work but you may be able to get to it from the cupboard. Take out the shelf and the pump is there.
The drain from the shower has a filter but it has big holes so all the dregs are sucked in.
You may need to undo all the elbows to find which one is blocked.
I use a filter stolen from the washing machine over the hole now and unscrew the outer filter from time to time to clean it. It's easier than stripping everything down the whole time.
Hope that helps a bit. BR Fred